Radio frequency (RF) communication systems have historically been extremely susceptible to the adverse effects of co-channel interference. For example, co-channel interference is caused by the transmission of a signal from a nearby source on the communications channel of concern. Conventional receivers suffer from the basic inability to accurately distinguish a desired signal from other signals transmitted by an interfering source, especially when the interfering signals are strong compared to the weaker signals to be detected.
Some RF receivers are required to operate over instantaneously wide bandwidths, however, they are limited in performance due to interfering signals in the background or from other systems that are close by. Some applications require detection of weak signals in the presence of strong interfering signals. Generally, as the bandwidth of a signal increases, it becomes more difficult to maintain the dynamic range of the receiver, that is, there is a tradeoff between the bandwidth and dynamic range of a signal.
In conventional telecommunications systems, analog to digital converters (ADCs) must receive and digitally convert RF signals over a given dynamic range of amplitudes. The RF receiver typically first filters the signals to remove unwanted signals and noise outside of the frequency range of interest, particularly to prevent aliasing in the sampling process. Therefore, while ADCs using such signal filtering are suitable for single air interface or single carrier applications, such ADCs are less capable of meeting the stringent requirements of wideband reception systems that receive more than one signal at a time, for example, multi-air interface, multi-carrier systems, such as software-defined radios, which must process RF signals over a large dynamic range. This is because ADC dynamic range is limited especially for wide bandwidths, although higher dynamic range ADCs exist for narrow band operation. Well known (interfering) signals can be cancelled but cancellation can be expensive and is imperfect, introducing artifacts, the lack of which is as important as the improvement in dynamic range obtained. The cost and difficulty increases as the number of interfering signals increases. Often, there may be bands of interfering signals resulting from licensing of spectrum to cell phone providers, TV stations, and other commercial and government entities who radiate many signals in limited bands of the spectrum.
Most of the existing solutions for signal cancellation are for narrow band receivers with extremely narrow band interference. Moreover, some prior systems have modeled steady state interference, used notch filters to block it out, or created feedback loops to adaptively remove signal content. Adaptively removing signals and cancelling signals generally involves delaying the input signal while a cancellation signal is created and applied. The delay introduces high latency to the receiver and is difficult to achieve, especially for wide band signals. Notching filters and other filtering schemes are undesirable and they fail to ensure that the wanted signals are retained for detection.